Transfer data between nodetrees in Animation Nodes
You can just store/get data from any node tree in memory as described in this answer. Alternatively, you can use the Data Interface node to pass data between node trees.

In a node tree, you can export some data using the Data Interface node, then, in another node tree, you can access that data by referencing the node tree name and the Data Interface node name using the following expression:

bpy.data.node_groups[nodeTreeName].nodes[nodeName].value


Where nodeTreeName is your node tree name and nodeName is the Data Interface node name. Here is an example:



As for the Go to parent nodetree operator, this has nothing to do with Animation Nodes. It is a native operator that allows one to exit a node group when using cycles or the compositor (Pressing Tab).